Protection Unit Support

APB protection unit support is provided by the optional PPROT[2:0] signals. The purpose is to let the Requester indicate protection information for the transaction, so that the interconnect or other system components can protect against illegal transactions.

PPROT is driven by the Requester. When implemented, it must be valid when PSEL is asserted.

PPROT[2:0] provides transaction protection information.

Bit	Protection	Description	LOW meaning	HIGH meaning
PPROT[0]	Normal or privileged	Indicates processing mode. A privileged processing mode typically has a greater level of access in the system.	Normal access	Privileged access
PPROT[1]	Secure or non-secure	Used in systems that require greater differentiation between processing modes.	Secure access	Non-secure access
PPROT[2]	Data or instruction	Gives an indication of data or instruction access. This indication is a hint and might not be accurate in all cases.	Data access	Instruction access

The primary use of PPROT is as an identifier for Secure or Non-secure transactions. It is acceptable for a system to use different interpretations of the PPROT[0] and PPROT[2] identifiers.

PPROT presence and compatibility

PPROT is optional on Requester and Completer interfaces. Compatibility depends on whether each side implements the signal.

Requester PPROT	Completer PPROT	Compatibility
Not present	Not present	Compatible
Not present	Present	Not compatible, unless fixed protection attributes are functionally correct
Present	Not present	Compatible; the Completer has no access protection, so PPROT can be ignored
Present	Present	Compatible

APB Protocol